Best Practices for Segmental Retaining Wall Design

The intent of this material is to communicate the best practices for design of Segmental Retaining Walls (SRW) as determined by Allan Block Corporation based on 25 plus years of research, design and field experience. This is not meant to be a final authority as each project has its own set of unique situations.

The local engineer of record must use their best engineering judgment to account for those situations that present themselves and provide a safe and efficient design for the customer. At no time does the contractor or local building official have the authority to override the approved plans and specifications provided from the local engineer of record.

It is the recommendation of Allan Block Corporation that the local engineer of record work for and be paid by the project owner. It has been determined that the local engineer of record should be the Project Site Civil Engineer as they are best suited to take responsibility for the design, and how it affects the site, whether they do the design in-house or use an outside consultant to do the design for the project.

The Project Site Civil Engineer has control of several of the overall aspects of the project and therefore is most able to properly handle the integration and communication required to ensure the performance of the wall complies with the needs of the site. For wall design applications that are outside of the experience level of the Project Site Civil Engineer, a wall designer with the appropriate knowledge and experience should be contracted with by the Project Site Civil Engineer. It is recommended that the wall contractor not be responsible for securing the engineering.

7.1 Certification to ensure proper materials for the geogrid reinforcement. Most geogrids used for soil reinforcement are manufactured from polyester fiber. To ensure a high quality product, use a geosynthetic reinforcement that has obtained an NTPEP Report.

There are many different international manufacturers that are supplying geogrid reinforcement, and they are not all equal. It is important to understand what influences the durability of the geogrid. The U.S. Federal Highway Administration identified three key factors:

Soils that have a pH of 10 or more represent an environment that could potentially degrade the geogrid faster especially in the presence of sufficient water.

Polyester molecular weight – the size of the polymer molecule has a significant influence on the chemical durability.

Polyester carboxyl end group (CEG) – grids are less susceptible to degradation when they have fewer CEG in their molecular structure.

Establish minimum requirements when using polyester geogrids. NCMA adopted the guidelines that the FHWA set for geogrid reinforcement. All polyester geogrids submitted for approval must be made of polyester fiber that meets the following specifications:

Molecular weight greater than 25,000 g/mol

Caboxyl End Group less than 30 mmol/Kg

The certification, obtained from the geogrid manufacturer, must originate from the actual manufacturer of the fiber to show conformance with the specification.

d) Designer must consider soils that have a pH of 10 or more represent an environment that could potentially degrade the geogrid faster especially in the presence of sufficient water and may require additional design criteria.

For straight walls, placement of adjacent sheets of geogrid should be side-by-side without any overlap. For walls with corners or curves the following are the best practice method of placement:

For outside corners, to avoid overlapping grid layers, grids should be stagger by one course on either side of the corner, weaving the strong direction from bottom to top of wall. See Allan Block standard detail 10 in Allan Block Spec Book.

For inside corner, extend the geogrid layer into the corner from one side a distance equal to 25% of the total wall height. Alternate extended geogrid layer from side to side per course to create a geogrid weave as wall is built. See Allan Block standard detail 9 in Allan Block Spec Book.

For outside curved and outside cornered walls, additional depth of wall rock should be specified to promote greater stability in these areas. Additional depth of wall rock is dependent on the total height of the wall and should be a minimum of H/2 as detailed in Drawing No. 6 on the top of this page.